Our overall objective is to characterize the chloride transport pathways in the cell membranes of a model intestinal epithelium, in order to gain some insight into role of these pathways in the regulation of transport and volume in normal tissues, and in the causes and maintenance of cystic fibrosis and secretory diarrheal diseases. We have chosen the amphibian gallbladder as the model since its absorptive function is similar in many respects to that of the small intestine, and since it can be changed to a secreting organ by stimuli (such as inflammation, prostaglandins, vasoactive intestinal peptide, cholera toxin) which also lead to a secretory response in the intestine. Furthermore, the gallbladder contains a chloride conductance pathway (whose activity may determine the degree of secretion) apparently similar to that seen in the intestine of similar species, but which is much more conveniently and easily studied owing largely to the simplicity of the gallbladder epithelium. We plan to characterize these apical chloride channels in physiological terms, to develop a monoclonal antibody to them, and to characterize the specific protein(s) responsible for the chloride conductance. Indeed, we have already developed a polyclonal antibody (by immunizing mice with Necturus gallbladder [NGB] cells) which binds to the apical membrane and inhibits chloride conductance. It is not unlikely that the apical chloride channel is similar in some respects to chloride channels in other epithelia; our recent studies indicate that toad (Bufo marinus) and mudpuppy (Necturus maculosus) gallbladders and mudpuppy small intestine have physiologically similar apical chloride pathways. If we succeed in isolating and characterizing the gallbladder channel, it will be of great interest to study its possible molecular similarity to chloride channels in other epithelia, such as those in intestine, pancreas, or trachea, all of which are affected in cystic fibrosis.